Laser radar apparatus having multiple output wavelengths

ABSTRACT

A coherent laser radar (lidar) device is described. The device has a transmitter portion that comprises a single wavelength laser source, a conversion means (such as an electro-optic modulator) for producing a combined light beam that comprises at least two component light beams of discrete wavelength from the output of said single wavelength laser source, and transmit optics to direct the combined light beam to a remote target. Each component light beam of the combined light beam traverses the same optical path from the single wavelength laser source to the transmit optics. The device is used to make differential absorption measurements.

This invention relates to laser radar (lidar), and in particular to anoptical fibre based differential absorption lidar (DIAL) system for realtime monitoring and measurement of gaseous species.

A variety of techniques for the measurement of gas concentrations arewell known. For example, a number of static sensors are commerciallyavailable that allow accurate (parts per billion) localised sampling ofa series of gas species. Many of these are “point sensor” devices whichdraw in a sample and analyse it using classical techniques, such as gaschromatography, flame ionisation or by Fourier Transform Infra-Red(FTIR) spectroscopy. These devices, by their nature, require significant(i.e. greater than is) sampling times and need to be located within thezone of interest.

Alternative techniques are also known which rely on non-dispersiveinfra-red (NDIR) or non-dispersive ultra-violet (NDUV) absorption.Typically, a series of absorption filters and a broadband source areused to provide light of a wavelength within the narrow absorption bandsassociated with a gas species of interest. Measurement of the absorptionlevel at the particular wavelength of interest provides a measure of thegaseous concentration, but such systems are typically only capable ofcapture rates as fast as half a second, and are prone to interferingspecies.

Differential absorption lidar is another known technique for the remotedetection of gas phase constituents. The basic concept of a DIAL systemis that two wavelengths of laser light are transmitted by the lidar(light detection and ranging) apparatus. The first wavelength is set ata discrete absorption line of the gas species of interest, whilst thesecond wavelength is set close to, but away from, the absorption peak.The differential absorption of the first and second wavelengths providesa quantitative measure of the average molecular concentration of the gasspecies. DIAL techniques thus have an advantage over the alternativetechniques described above in that they permit remote detection.

DIAL systems have been implemented in numerous ways; for exampleanalogue, photon-counting and coherent detection systems are known.Coherent detection (or heterodyne) DIAL systems, in which the returnradiation is coherently mixed with an optical local oscillator beam,typically provide a high signal to noise ratio with a good degree ofimmunity to interference from background light and cross-talk. Coherentdetection DIAL systems are described in more detail elsewhere; forexample see Rye, Appl. Opt. 17, 3862-3864 (1978).

A disadvantage of known coherent DIAL systems is that any fluctuationsdue to speckle in the return light will lead to a large uncertainty inthe detected optical powers unless the received signal is averaged overmultiple speckle correlation times. The numerical averaging required toovercome the effects of intensity fluctuations therefore limits thespeed at which the differential absorption data can be extracted fromthe DIAL system.

Recently, a coherent DIAL system has been demonstrated by Ridley et al(see Applied Optics, Vol. 40, No. 12, pp 2017-2023, 20 Apr. 2001) thatcan reduce the time over which received signals need to be averaged byup to an order of magnitude. The device comprises a pair of laser diodesthat are used to produce radiation of two slightly differentwavelengths. Local oscillator beams are extracted, and the two differentwavelength laser beams are then combined and coupled into a singleoptical fibre. After combination the two different wavelength beamsshare a common path through the remaining portion of the transmitter, infree space to/from the target and in the receiver. For small wavelengthseparations and a shallow target depth, the device exhibitswell-correlated speckle fluctuations for the two wavelength channels.

A disadvantage of the system described by Ridley et al is the existenceof intensity variations that arise separately in each of the twodifferent wavelength channels prior to beam combination in thetransmitter. The presence of such uncorrelated variations increases thetime over which the measured intensity signals must be averaged toprovide reliable differential absorption data. Alternatively theincreased requirement for data averaging can be considered to limit therate at which differential absorption information having a certainsignal to noise ratio can be obtained.

Co-pending PCT patent application GB03/003882 (based on GB applicationnumber 0220914.6) describes how the device described by Ridley can beenhanced by providing an additional normalisation signal. However, theimproved performance is achieved at the expense of additional devicecomplexity.

It is an object of the present invention to mitigate at least some ofthe disadvantages described above.

According to a first aspect of the present invention a coherent laserradar (lidar) device has a transmitter portion comprising a singlewavelength laser source, a conversion means for producing a combinedlight beam that comprises at least two component light beams of discretewavelength from the output of said single wavelength laser source, andtransmit optics to direct the combined light beam to a remote target,wherein each component light beam of the combined light beam traversesthe same optical path from the single wavelength laser source to thetransmit optics.

The present invention thus provides a coherent lidar device in which thetwo or more component light beams produced for direction to a remotetarget are provided by the same laser source and travel the same opticalpath through the transmitter portion. This provides a device in whichthe majority of noise (e.g. variations of the laser output or noiseintroduced by the various optical components of the transmitter portion)is common to both the wavelength channels and therefore can be correctedout from the differential absorption information acquired by the device.Furthermore, only a single local oscillator signal is required from thesingle wavelength laser source to enable coherent detection of anyreturned signal.

The remote target may comprise a scattering or reflective target that isplaced a short distance (e.g. metres) or several kilometers from thedevice. A skilled person would also recognise that, with an appropriatedesign of optics, the system could be readily configured to makemeasurements at much shorter ranges. The scattering or reflective targetmay be a purpose made scatterer a scatterer of opportunity.Alternatively, the remote target may be an aerosol. A person skilled inthe art would recognise the various optical arrangements that would berequired to provide a collimated or focussed beam forreflection/scattering from a solid remote target, and how a focussedbeam could be obtained to allow measurements from defined targetvolumes.

It should be noted that the term “light” as used herein includesradiation in the ultraviolet, visible or (near/mid/far) infra-red. Asdescribed in more detail below, a person skilled in the art wouldrecognise that a device of the present invention could be implementedacross a wide range of wavelengths and in the case of a differentialabsorption lidar (DIAL) device the precise wavelengths of operationwould be selected on the basis of the gas species of interest.

Conveniently, a receiver portion is additionally provided that comprisesreceive optics to collect light returned from the remote target and acoherent detection means.

Advantageously, each component light beam collected by the receiveoptics traverses the same optical path from the receive optics to thecoherent detection means.

In this case it can be seen that the beams forming the combined beamwill be almost totally co-linear; i.e. the beams will follow a commonoptical path within and outside the device. This minimises instrumentaldrift that only affects some of the wavelength component beams of thecombined beam, thereby reducing the noise associated with any resultingdifferential absorption data. The device thus gives more robustnessagainst noise sources such as speckle, and provides a reduced rate offalse alarms.

Preferably, the conversion means comprises an electro-optic modulator(EOM).

An EOM provides a convenient way of producing the two or more componentlight beams of discrete wavelength. EOM devices are commerciallyavailable; for example, a suitable device is described on page 78 of the2002/2003 catalogue (Vol. 11) produced by New Focus Inc., 5215 HellyerAve, San Jose, Calif. 95138-1001, USA.

Conveniently, the EOM is electrically driven to provide at least threecomponent light beams of discrete wavelength.

Advantageously, the EOM is electrically driven to provide at least fivecomponent light beams of discrete wavelength.

The use of three or five or more discrete wavelength channels overcomesthe possible opacity effect that can be observed when only twowavelength channels are used. This is described in more detail belowwith reference to FIG. 4.

Preferably, the transmitter portion additionally comprises apolarisation control means.

The EOM will have an radiation input polarisation at which the amount ofpower converted in to higher order modes is maximised. A polarisationcontrol means may thus be used to adjust the polarisation of light tomaximise EOM conversion efficiency. The device may further oralternatively comprise a polarisation control means configured tocontrol the polarisation state of the received light and/or theextracted component light beam with respect to the polarisation state ofthe associated local oscillator beam. This allows the heterodyne mixingefficiency at the heterodyne detection means to be controlled (e.g.maximised) by matching the polarisation of the signal and localoscillator beams.

The transmit portion may advantageously further comprise at least oneoptical amplifier. For example, an erbium doped fibre amplifier (EDFA).The additional of an amplifier enables output signal strength to beincreased if the remote target has a low scattering strength (e.g.because it is a distributed target such as an aerosol).

Conveniently, a frequency shifting means is provided to introduce afrequency shift between the laser beam received by the conversion meansand its associated local oscillator signal. This enables thecontributions to the signal at the different transmitted wavelengthsreturned from the target to be readily separated.

Preferably at least some of the optical components of the device areinterconnected via optical fibre cable. Although the invention ispreferably implemented using a fibre optic based device, it should berecognised that some or all of the internal optical components of thedevice may be optically linked by light beams transmitted through freespace. For example, a lens used to transmit the combined beam mayreceive such light through free space from a fibre end that is locatedan appropriate distance from the lens.

The width of the detected heterodyne peak may be broadened if the pathlength (i.e. device to target to device) is significantly greater thanthe coherence length of the laser. The addition of a delay line to delaythe local oscillator beam will reduce this unwanted peak broadening. Thelocal oscillator beam is thus advantageously coupled from thetransmitter portion to a receiver portion via an optical fibre delayline.

Preferably, separate transmit optics and receive optics are provided. Inother words, the optical components (lenses etc) used to transmit thebeam to the remote target are separate to the optical components thatare used to collect the returned beam; i.e. the arrangement is bistatic.It should be noted that it is also possible to implement the presentinvention using common transmit and receive optical components; i.e. amonostatic transceiver configuration.

Advantageously, the wavelength of one of the at least two componentlight beams is selected to coincide with a peak in absorption of a gasspecies of interest.

Conveniently, the device further comprises means to vary the wavelengthof one or more of the at least two component light beams in response tothe detected return signal falling below a threshold level. In otherwords, if a wavelength channel becomes opaque due to the presence of ahigh concentration of gas species the transmitted wavelength can bealtered so that it falls on a different region of the peak in absorptionof the gas species of interest. This increases the dynamic range of thedevice as described below with reference to FIG. 4.

According to a second aspect of the present invention a lidar devicecomprises a single laser source and arranged to transmit a beam thatcomprises three or more wavelength components to a remote target.

Conveniently the device comprises an electro-optic modulator.

The invention will now be described, by way of example only, withreference to the following drawings in which;

FIG. 1 shows a prior art fibre based DIAL device,

FIG. 2 illustrates an DIAL device according to the present invention,

FIG. 3 illustrates the relative intensities and wavelengths of thesignals produced by a device of the type shown in FIG. 2 and

FIG. 4 demonstrates how a device of the present invention can be used tomake differential absorption measurements.

Referring to FIG. 1, prior art coherent DIAL apparatus of the typedescribed by Ridley et al is shown. The apparatus comprises atransmitter portion 2, and a receiver portion 4.

The transmitter portion 2 comprises a first laser 6 and a second laser8. The first and second lasers are distributed feedback (DFB)semiconductor lasers that operate within a user selectable wavelengthrange of 1500 nm to 1600 nm and have a linewidth of 20 KHz.

The optical output of the first laser 6 is fed into a firstbeam-splitter 10 via an optical fibre. The optical output of the secondlaser 8 is fed into a second beam-splitter 12 via an optical fibre. Thefirst and second beam-splitters 10 and 12 divide each beam they receiveinto a local oscillator beam and a main beam. The polarisation of eachlocal oscillator beam is adjustable using fibre polarisation controllers14 and 16 to ensure maximum heterodyne mixing efficiency is obtained inthe receive portion.

The two main beams output by the first and second beam-splitters 10 and12 are fed, via optical isolators 18 and 20, to first and secondacousto-optic modulators (AOMs) 22 and 24 respectively. The AOMs 22 and24 introduce a phase shift between each main beam and its associatedlocal oscillator beam to enable subsequent coherent (heterodyne)detection in the receive portion. A 79 MHz frequency shift is impartedby the first AOM 22, and an 81 MHz frequency shift is imparted by thesecond AOM 24.

The frequency shifted beams are then combined by beam combiner 26 andcoupled into a single optical fibre. The combined beam is transmitted toa remote target via transmission optics 28. A portion of the combinedbeam is directed, prior to transmission to the remote target, to a laserwavelength monitor 30 to permit the wavelengths of the components of thetransmitted beam to be continually measured.

The receiver portion 4 comprises receive optics 32 that collect anyradiation returned from the remote target. The return signal is coupledinto a single optical fibre cable, mixed with both local oscillatorbeams that have been combined by beam combiner 17, and fed to a detector34 and a monitor 36 via a beam combiner/splitter means 38.

Heterodyne mixing at the detector 34 of the return beam and theassociated local oscillator beams produces two signals at frequenciescorresponding to the difference in frequency of each component of thereturned beam and its local oscillator. In this case, a signal at 79 MHz(i.e. the phase shift imparted by the first AOM 22) and a signal at 81MHz (i.e. the phase shift imparted by the second AOM 24) is detected atthe detector 34. The electrical signals produced by the detector 34 areintegrated over a narrow bandwidth by a spectrum analyser means 40 thatprovides electrical output signals indicative of the strength of both ofthe wavelength components of the returned beam. In this manner,differential absorption data can be obtained.

It can be seen that the two laser sources 6 and 8 produce beams that areof a similar wavelength and, after beam combination in the transmitterportion, the beams pass along the same optical path until detection.Therefore, as described previously by Ridley et al (ibid), any noise dueto atmospheric disturbances and/or Pointing instability will be similarfor the two different beams and have no significant effect on themeasured differential absorption ratio.

Although the two beams pass along the same optical path aftercombination and prior to detection, there still remains a significantamount of noise introduced prior to beam combination in the beamcombiner 26. The noise, for example fluctuations in laser intensity andnoise introduced by each AOM, introduces uncertainty into the measureddifferential absorption data. Although additional normalisationtechniques (for example that described in co-pending PCT applicationGB03/003882) can be employed to improve the quality of the measuredifferential absorption signal, they tend to involve additional devicecomplexity and/or additional signal processing.

Referring to FIG. 2, a fibre based DIAL device according to the presentinvention is shown. The apparatus comprises a transmit portion 50 and areceive portion 52 but, unlike the DIAL device described with referenceto FIG. 1, the transmit portion 52 of a device of the present inventioncomprises only a single laser source 54.

A local oscillator beam is tapped from the output of the laser sourceusing a beam splitter 56 and passed to the receive portion 52 whilst theremaining power of the laser beam is shifted 80 MHz in frequency by anAOM 58. The optical output of the AOM 58 is then fed, via a firstpolarisation controller 60, to an Erbium doped fibre amplifier (EDPA)62. The amplified optical signal produced by the EDFA 62 is passed to anelectro-optic modulator (EOM) 64. A first polarisation controller 60 isarranged so as to provide a signal having a polarisation that matchesthe input polarisation required for optimum performance of the EOM 64.

As described in more detail with reference to FIG. 3 below, the EOM 64converts the single frequency beam it receives into a number ofcomponent beams of different frequency. The beam output by the EOM thuscomprises a number of collinear beams of different wavelength which forma so-called combined beam (i.e. a beam that is made from the combinationof a number of beams of different wavelength) that is transmitted viathe transmission optics 66 to a remote target (not shown).

It should be noted that the frequency of light output by the laser 54 ina device of the present invention may be stabilised using a feedbackloop. A small proportion of the light output by the laser source 54could be fed to a feedback means 55. The feedback means 55 is configuredto analyse the laser output and to alter the intensity and/or wavelengthof the laser source 54 in order to maintain the required laser output.

The receive portion 52 comprises receive optics 68 to collect anyradiation returned from the remote target and passes the received beam,via an optical fibre, to a beam combiner/splitter means 70 where it iscombined with the local oscillator beam derived from the laser source 54and fed to a balanced receiver detection system comprising a firstreceiver 70 and a second receiver 72. The efficiency of the mixing atthe detectors is optimised by controlling the polarisation of the localoscillator beam using a second polarisation controller 74.

A balanced receiver detection system of this type has variousadvantages, albeit with an associated increase in cost and complexity,over the detection system described with reference to FIG. 1. Forexample, a balanced receiver detection system slightly increases thedetected signal strength and reduces the effect of laser intensity noise(RIN). A skilled person would recognise that any appropriate heterodynedetection means could be used in a lidar device of the presentinvention.

The EOM 62 splits the single wavelength beam produced by the laser intoa number of component beams each having a different wavelength asrequired, and FIG. 3 shows the spectral output of the EOM used in thedevice described with reference to FIG. 2.

It can be seen from FIG. 3 that the EOM 62 outputs a zeroth order beamJ₀ at a frequency ν−δ where ν is the frequency of the laser source 54and δ is the frequency shift imparted by the AOM 58. First order beamsJ₁ and J⁻¹ having an equal intensity are also produced at frequenciesν−δ+Δ and ν−δ−Δ respectively where A is the first order frequency shiftimparted by the EOM and would typically be around 500 MHz. Second orderbeams J₂ and J⁻², of a different frequency and of a lower intensity thanthe first order beams, are also output.

As the local oscillator beam has a frequency ν (i.e. it is tapped fromthe transmission portion prior to application of a frequency shift bythe AOM 58), each of the signals J₀, J⁻¹, J₊₁, J⁻², J₊₂ etc will producea corresponding heterodyne signal at the detectors 70 and 72 of a uniquefrequency. A spectrum analyser 78 is used to integrate the electricalsignals produced by the detectors 70 and 72 over a narrow bandwidth inorder to provide electrical output signals indicative of the strength ofeach of the signals J₀, J⁻¹, J₊₁, J⁻², J₊₂ etc. In this manner,differential absorption data can be obtained. Any systematic distortionof the differential absorption ratio (e.g. systematic errors arising dueto non-uniform frequency response of the detectors) can be removed usingstandard calibration techniques. Although a spectrum analyser 78 isshown, it would be recognised that band pass filters combined with anelectrical power meter could be used as an alternative.

It should be noted that the electrical power applied to the EOM 62defines the proportion of optical power that is frequency shifted to thehigher order beams from the zeroth order beam, whilst the frequency ofthe RF signal applied to the EOM defines the magnitude of the frequencyshift Δ. The use of an EOM thus provides a means of controlling thewavelengths of the component beams that are generated in thetransmission portion of the device.

As described above, the EOM 62 operates most efficiently when itreceives light of a certain polarisation and hence the use of the firstpolarisation controller 60. However, it should be noted that inputting anon-optimum polarisation to the EOM only reduces the efficiency withwhich it frequency shifts radiation into the higher order beam; i.e. theratio of the optical power in, say, each of the first order beamsremains constant.

Referring to FIG. 4, additional advantages of using a device of thepresent invention are shown.

In conventional DIAL lidar devices, two beams of slightly differentwavelength (e.g. λ₁ and λ₂) are used. One wavelength (e.g. λ₁) is chosento coincide with an absorption peak (e.g. the transmission trough 90)and the other wavelength (e.g. λ₂) is selected to be away from, or atthe edge of, that peak. The relative difference in absorption ofwavelength λ₁ compared to λ₂ provides an indication of the concentrationof the gaseous species of interest. However, if a large amount of thegas species is present virtually all of the radiation at wavelength λ₁will be absorbed by the gas. This “bottoming out” or opacity of the λ₁channel effectively places an upper limit on the concentration of thegas that can be accurately measured; i.e. it reduces the dynamic rangeof the device.

A device of the present invention permits the opacity effect observedwith dual wavelength lidar systems to be overcome by simultaneouslymeasuring the absorption at a number (e.g. five) of discretewavelengths. For example, taking the system described with reference toFIGS. 2 and 3, the five beams of different wavelength (J₀, J⁻¹, J₊₁,J⁻², J₊₂) output by the device can be spread across the absorption peakof the gas species of interest as shown in FIG. 4. In this case, if thesignal returned from beam J⁻² becomes opaque, the absorption of theother beams near the absorption peak (e.g. beams J⁻¹, J₀ and J₊₁) canstill be measured relative to the J₊₂ beam.

In an alternative configuration the device could be arranged to normallyoutput three beams of different wavelength (e.g. J₀ and J_(±1)), butcould also comprise a means of increasing the power applied to the EOMto provide higher order beams (e.g. J_(±2)) if any of the three initialsignals become opaque. A device of the present invention can thus beseen to have a greater dynamic range, and to be generally more flexible,than dual wavelength DIAL systems.

A person skilled in the art would recognise the various alternativeoptical arrangements that could used to implement the present invention.For example, the various optical components making up the transmissionportion could be altered in order and still provide a system thatprovides substantially the same function. The feedback mechanism used tostabilise the output of the laser source could also be changed so as touse light tapped out of the main optical path at any of a variety ofpoint, such as after the EOM.

The apparatus could also be configured to measure the differentialabsorption properties of a gaseous species that is moving relative tothe DIAL device. For example, the apparatus could be operated from amoving aircraft or vehicle. In such a case, a skilled person wouldrecognise the Doppler shift effects due to the relative motion thatwould also need to be considered to ensure proper operation of the DIALdevice.

Although bistatic transceivers (i.e. transceivers having separatetransmit and receive optics) are described above, a person skilled inthe art would recognise that monostatic transceivers (i.e. transceivershaving combined transmit and receive optics) could be used. Monostaticdevice would be particularly useful when using a pulsed transmit beam.Similarly, it would be recognised by a skilled person that althoughoptical fibre based systems are preferred for many reasons (e.g. ease ofcomponent alignment, cost etc) the present invention could also beimplemented using free space optical components.

The device described above employs a laser that emits radiation in the1500 nm to 1600 nm range. However, the invention could be implementedusing radiation of any wavelength. A skilled person would recognise thatthe wavelength of the laser source would simply be selected so that theoutput radiation matches the absorption maxima of the gas species ofinterest. As laser diode technology develops, it will thus becomepossible to cheaply access wavelengths further into the infrared usingthis technique. The use of an increased wavelength may be a considerableadvantage for differential absorption measurements of species such ascarbon monoxide, nitrogen oxides and unburned hydocarbons.

1. A coherent laser radar (lidar) device having a transmitter portionthat comprises a single wavelength laser source, a converter forproducing a combined light beam that comprises at least two componentlight beams of discrete wavelength from the output of said singlewavelength laser source, and transmit optics to direct the combinedlight beam to a remote target, wherein each component light beam of thecombined light beam traverses the same optical path from the singlewavelength laser source to the transmit optics.
 2. A device according toclaim 1 wherein a receiver portion is additionally provided thatcomprises receive optics to collect light returned from the remotetarget and a coherent detector.
 3. A device according to claim 2 whereineach component light beam collected by the receive optics traverses thesame optical path from the receive optics to the coherent detector.
 4. Adevice according to claim 1 wherein the converter comprises anelectro-optic modulator (EOM).
 5. A device according to claim 4 whereinthe EOM is electrically driven to provide at least three component lightbeams of discrete wavelength.
 6. A device according to claim 4 whereinthe EOM is electrically driven to provide at least five component lightbeams of discrete wavelength.
 7. A device according to claim 4 whereinthe transmitter portion additionally comprises a polarisationcontroller.
 8. A device according to claim 1 wherein the transmitportion further comprises at least one optical amplifier.
 9. A deviceaccording to claim 1 wherein a frequency shifter is provided tointroduce a frequency shift between the laser beam received by theconverter and its associated local oscillator signal.
 10. A deviceaccording to claim 1 wherein at least some of the optical components ofthe device are interconnected via optical fibre cable.
 11. A deviceaccording to claim 10 wherein the local oscillator beam is coupled fromthe transmitter portion to a receiver portion via an optical fibre delayline.
 12. A device according to claim 1 wherein separate transmit opticsand receive optics are provided.
 13. A device according to claim 1wherein the wavelength of one of the at least two component light beamsis selected to coincide with a peak in absorption of a gas species ofinterest.
 14. A device according to claim 13 wherein the wavelength ofat least one of said at least two component light beams is varied whenthe detected return signal falls below a threshold level.
 15. A lidardevice comprising a single laser source wherein said device is arrangedto transmit a beam that comprises three or more wavelength components toa remote target.
 16. A device according to claim 15 that comprises anelectro-optic modulator.